The present invention is generally directed to combustion, and more specifically to a method of operating a main burner wherein the main combustion occurring therein is supported by a catalytic pilot that oxidizes a fuel rich mixture and a main burner for use therewith.
Power is generated in a gas turbine engine by the expansion of heated gases against a rotating turbine. To accomplish this heating and expansion a gas turbine has at least one combustor having at least one main burner positioned therein. The main burner combines a fuel and air into a fuel/air mixture and combusts the mixture thereby creating the expanding hot gases. Combustion of the mixture generally occurs by a flame mechanism.
A problem commonly associated with the operation of gas turbines employing a flame mechanism is that at high flame temperatures, particularly above 2800 degrees F., oxygen and nitrogen present in the air combine by a thermal formation mechanism to form pollutants such as NO and NO2, collectively referred to as NOx. In a gas turbine, temperatures of most common fuels combusting in air can easily exceed this value. Accordingly, it has been an objective of gas turbine combustion system designers to develop methods and associated apparatuses for combustion that produce reduced temperatures at or below 2800 degrees F., so that such thermal formation of NOx is limited.
Modern combustion methods employed in gas turbine combustors reduce flame temperatures, and thereby NOx, by using excess air to create lean fuel/air mixtures, e.g. mixtures that contain more air than needed to fully combust all the fuel present. Quantitatively, the mixture has a fuel/air equivalence ratio less than one. The equivalence ratio is the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio, where the stoichiometric coefficients are calculated for the reaction giving full oxidation products CO2 and H2O. An equivalence ratio greater than one defines a fuel-rich fuel/air mixture, and an equivalence ratio less than one defines a fuel-lean fuel/air mixture. For any given substantially premixed fuel/air mixture, the combustion temperature will be at its highest temperature when the fuel/air mixture being combusted has a fuel/air equivalence ratio of about one.
The more excess air added to and well mixed in a fuel/air mixture, the leaner the resulting fuel/air mixture becomes and the lower the flame temperature of that mixture. However, if too much excess air is added the resulting fuel/air mixture will become so lean that it will not homogeneously combust. In this situation, the mixture is said to have reached its lower flammability limit. Therefore, excess air to limit flame temperature can only be added to a well mixed fuel/air mixture until this limit is reached.
In order to obtain the benefits of lower flame temperatures in fuel/air mixtures, the fuel/air mixture being combusted must be substantially mixed. Typically, the lower the unmixedness the lower the NOx that will be produced. While unmixedness defines a continuum such that mixtures can only be categorized as being mixed to some degree, a xe2x80x9csubstantially premixed mixturexe2x80x9d can be defined based on the fuel/air mixture""s potential to produce a certain level of NOx when combusted within the context of acceptable NOx production based on existing environmental regulation. In other words, the mixture is mixed sufficiently to produce a level of NOx that will meet current environmental regulations.
Thus based on current environmental regulation, substantially premixed fuel/air mixtures are mixtures wherein the average variation of fuel/air ratio from the mean is less than about 20 percent of the mean value and more preferably in the range from about 10 percent to about 2 percent, with less than 2 percent being a practical minimum. Mean fuel/air ratio refers to the average fuel/air ratio as measured at various points in the region of interest. Variation from the mean refers to the magnitude of the difference between the mean and the measured fuel/air ratio at some single measured point, and the average variation from the mean is the average of all measured variations from the mean. For a combustible fuel/air mixture the region of interest is generally immediately prior to combustion.
In a combustor, the air stream and the fuel stream must form a fuel/air mixture prior to combustion. To mix two flowing fluid streams to form a single flowing stream, the individual streams must be brought into contact and travel some distance together. If mixing is done within a duct, the length of the duct will determine the degree of unmixedness. Generally speaking, the longer the duct the lesser the degree of unmixedness.
As a lean fuel/air mixture is made ever leaner but above the mixture""s lower flammability limit, the rate of combustion associated with the mixture decreases, i.e. the flame is becoming less robust. In order to maintain the flame, the environment within the flame must be made ever more conducive to combustion, e.g. the flow velocity must be reduced, otherwise the flame could be blown out, much like one blows out a candle. In a gas turbine when the fuel/air mixture has been leaned to the point that the rate of combustion of the mixture is too low to sustain combustion under the existing conditions, the extinguishing of the flame by its environment is termed blowout. Flame anchoring, i.e. the ability to provide proper environmental conditions to support a flame, and flame stability thus become problematic for fuel-lean combustion.
The management of combustion within a gas turbine operating on lean fuel/air mixtures to avoid blowout and assure flame anchoring and stability is complex. Gas turbines are generally designed to operate at a given or peak condition, i.e. an optimum condition which is highly efficient. However, during startup or at other times, it may be desirable to operate at other, or off-peak, conditions. Therefore, a gas turbine must have the ability to transition from the peak condition to off-peak conditions. This ability to go from a peak to off-peak condition is generally referred to by those skilled in the art as the ability to turndown the gas turbine.
Turndown is accomplished by reducing the fuel supply to the combustor, thereby making the fuel/air mixture being combusted therein leaner. As the gas turbine at its peak condition is already operating with a fuel/air mixture that is quite lean to meet current environmental standards, when the fuel/air mixture is made ever leaner to achieve the desired off-peak operating condition, sustaining combustion within the combustor becomes ever more problematic. In some cases, turndown is simply insufficient to permit acceptable off-peak operation conditions.
To increase the ability of a gas turbine to turndown, pilots can be used to support combustion within the combustor. Specifically, the pilots are supporting what is termed main combustion. Pilots that use flames operate at very favorable fuel/air mixtures, which may even be at fuel/air ratios at or near 1.0, providing highly stable and high temperature flames. Initially, pilot emissions were a small percentage of the overall emissions from the gas turbine. Currently, however, gas turbines have main combustion occurring at such lean fuel/air mixtures that NOx discharge is acceptable, and it is the emissions from these flame based pilots that must be further reduced to reduce overall gas turbine NOx emissions.
Conventional catalytic pilots on the other hand are highly stable but operate at lower temperatures, because of catalyst material considerations, thereby producing less NOx than flame pilots. However, these lower temperatures hamper the ability of the catalytic pilot to support combustion of lean fuel/air mixtures.
Based on the foregoing, it is the general object of the present invention to provide a method and apparatus for use therewith to support main combustion that overcomes the problems and drawbacks of the prior art.
The method of combustion utilizes catalytic oxidation to support flame burning of a lean fuel/air mixture. In the method of the invention a first, second, and third air, and a first and second fuel are provided such that: the first fuel and first air have a fuel/air equivalence ratio greater than 1; the first fuel in combination with the first air and second air have a fuel/air equivalence ratio less than 1; and the second fuel and the third air have a fuel/air equivalence ratio less than 1.
The first fuel and first air are introduced into a first common area so the first fuel and first air can travel together and intermix to form a first mixture. The first mixture is then flowed over and brought in contact with a catalyst where the fuel in the first mixture is oxidized resulting in creation of a first reacted mixture and a heat of reaction. The first reacted mixture is then introduced to the second air in a second common area so the first reacted mixture and the second air travel together and intermix to form a second mixture. The second mixture is then combusted. Combusted as used herein means that the mechanism of burning is a flame. The second mixture, which is derived from the first fuel, the first air, and the second air, is lean because the constituents from which it is derived in combination are lean. It should be understood as explained above, the second mixture can not be so lean as to be below the lower flammability for the second mixture.
Simultaneously, the third air and the second fuel are introduced into a common area where the third air and second fuel travel together and intermix forming a third mixture. The third mixture is then combusted. The third mixture is also lean because the second fuel and third air from which the third mixture was derived have a fuel/air equivalence ratio less than 1. As with the second mixture, it is understood that the third mixture must have a fuel/air equivalence ratio above the lower flammability limit of the third mixture.
The method requires that the combusting second mixture be in contact with the combusting third mixture. The second mixture is combusted in contact with the third mixture when the two flames interact. In other words, the flames touch. It is preferred that the flame of the second mixture be substantially within the flame of the third mixture.
In an enhancement to the method, a heat of reaction generated during the catalytic reaction of the first mixture can be transferred into the second air. Dissipating excess heat into the second air stream can protect the catalyst used in the oxidation of the first fuel mixture, i.e. backside cooling the catalyst. Backside cooling a catalyst protects the catalyst and substrate on which it might be positioned from damage from the extreme temperatures generated in exothermic catalytic oxidation.
While the first, second and third airs are identified as separate airs; the airs could be from a common source. In addition while the first fuel and second fuel could be different, the two fuels preferably are the same.
When this method is applied to a gas turbine, the method permits a pilot to produce less NOx than other standard flame pilots, but also provides a flame that is generally equally robust to that of flame pilots to support the main combustion. It is preferred that the combustion of the second mixture support, i.e. pilot, the combustion of the third mixture. As such, the ratio of first fuel to second fuel should be less than about 1:1 but greater than about 1:19. Preferably, the ratio should be less than about 1:4 and greater than about 1:9. These ratios permit this method to be employed within current gas turbine designs.
A ratio greater than 1:9 is preferred because the pilot can provide significant stability to the main combustion with pilot emissions being a small percentage of the overall emissions. At ratios greater than 1:1, the flows through the pilot can disrupt main combustion and overall pilot size to accommodate the flows therethrough become problematic.
A main burner in one aspect that can employ the above method comprises a catalytic pilot comprised of a first duct and a main mixer disposed within an interior area of a housing with the main mixer in fluid communication with a second duct with both the first duct and the second duct having exits positioned relative one to the another such that the exits cooperate to position a flame emanating therefrom in contact. More specifically, the catalytic pilot is comprised of a catalytic reactor in fluid communication with the first duct that has a first entrance, a second entrance, and an exit. The first entrance and second entrance are positioned coincident one with the other, or the first entrance is spatially upstream. The third duct is in fluid communication with the second entrance.
The first and second ducts have geometry to permit mixing to occur. As indicated above, mixing requires some finite length of the duct regardless of other geometric considerations. This length is a critical parameter that must be sufficient to permit the degree of mixing required by the application. In the second duct a fuel and oxidizer is mixed, and the first duct a reacted mixture and another oxidant is mixed. In terms of the method above, the first air and first fuel are mixed in the first duct to form the first mixture, and the first reacted mixture and the second air are mixed in the second duct to form the second mixture.
Mixing of the fluids within the ducts can be accomplished by any means such as entrainment or swirling. As those skilled in the art will appreciate, some mixing methods will require additional structure, such as swirlers, in the duct and other mixing methods such as entrainment will not.
Preferably, the catalytic pilot is positioned within a passage defined by the main mixer. In some applications, it might be desirable to make the main mixer an annulus and position the catalytic pilot within the vacant center region concentric therewith. When the catalytic pilot and the main mixer are positioned in this manner, the exit from the catalytic pilot should be spatially downstream from the exit of the main mixer.
The catalytic reactor within the catalytic pilot can be of almost any design. As discussed above, an additional step in the method is the transfer of some of the heat of reaction into the second fluid. In terms of the catalytic reactor, this step can translate into a backside-cooled catalyst. Backside cooling of a catalyst occurs where a catalyst, i.e. substance that promotes the desired reaction, is positioned on just one side of a two sided substrate and the catalytic reactor is designed to permit a flow of a fluid over both sides. This structure permits the heat generated by the exothermic reaction of the fuel/air mixture on the surface of the substrate having the catalyst to be conducted through the substrate to the other side and transferred into the fluid flowing in contact therewith.
The method and main burner could be used within the combustor of a gas turbine as well as other devices such as heaters. While the invention is discussed in the more conventional terms of fuel/air, the invention should not be considered so limited as any fuel and associated oxidant could be used.